Added extra constructors to selectively prevent ExpressionManager from instantiating ExpressionFactoryImpl 

Added extra constructors to selectively prevent ExpressionManager from instantiating ExpressionFactoryImpl on init. This allows overriding ExpressionManager/ExpressionFactory to be able to run Activiti on Google App Engine.

Please refer to http://forums.activiti.org/content/running-activiti-google-app-engine for more details.